bootable/copied volume is 5 GB smaller than the source -- normal?
 

My internal/source volume is using about 92 GB and my external/boot volume (all files backup) ended up using about 87 GB. I know the description explains that everything will be copied, "except the temporary and system-specific files that Apple recommends excluding," but is 5 GB the average size of "unnecessary" files?

It can be, yes: VM swap files and your 'sleepimage' can be quite large. The 'ignored' files are typically listed in the log if you'd like to review them.

That was fast. Thank you!